The overall objectives of this grant application are: 1. To use high precision isotope ratio mass spectrometry to determine the abundance of 13CO2 in respiratory CO2 after the administration of 13C labeled substrates to neonatal pediatric and obstetric patients. These non-radioactive compounds are chosen for their possession of a target bond whose cleavage liberates a 13CO2 producing functional group and whose rate or extent of metabolism to 13CO2 can be used as a non-invasive diagnostic measure of the presence or absence of disease. These techniques have been developed specifically for studies in those populations where radioactive tracers are contraindicated and the substrates chosen have previously demonstrated utility as 14C labeled forms. These include trioctanoin, tripalmitin and triolein as measures of fat malabsorption, lactose as a measure of lactase deficiency and aminopyrine as a measure of liver microsomal mass. 2. To evaluate and implement the use of an automatic sample inlet system that permits unattended isotopic ratio measurements. This system, under microprocessor control conducts sample purification, compensates sample and reference inlet pressures, measures isotopic abundances, calculates isotope ratio values and prints final information. Measurement of respiratory 13CO2 abundance will be augmented by measurements of total 13C recovery obtained by combustion of fecal and urinary fractions and by on-line combustion of gas chromatographic samples prior to isotope ratio measurements of the CO2 produced. 3. To map the absolute abundances of 13C in dietary constituents of formulls used in oral, intravenous and hyperalimentation formulas as baseline values for feeding enriched compounds. These will be coupled with continuing studies of 13CO2 abundance values as a function of endogenous substrates, physiological status and CO2 production in health and disease.